The present invention relates to a method for transferring workpieces from one supporting table to another, and in particular to an air float transfer system wherein the workpiece is transferred from one air float table to another. Although the system is adapted for use in a variety of applications, it is most particularly suited for use in a machine tool environment wherein it is necessary to move parts in and out of a number of machining and forming stations.
In the past, the machining of large workpieces involved very difficult positioning and repositioning of the workpiece when the various portions thereof are machined. In many cases, it is necessary to use hoists for elevating and moving the workpiece about both to and from the machine tool table, and on the table itself where repositioning is necessary for sequential machining steps.
In order to overcome these problems, an air float system, wherein the workpiece is mounted to a fixture which in turn is supported on a film of pressurized air, has been developed. This system is described in detail in U.S. Pat. No. 4,053,885 in the name of Raymond A. Bergman, which patent is incorporated herein by reference. In the system, a table is provided with fluid passages and a plurality of fluid outlets distributed over the surface of the table so that a cushion of pressurized air may be provided underneath the workpiece fixture. By virtue of the fluid pressure film, substantially friction-free movement of the fixture on the table is possible thereby permitting positioning and repositioning to be accomplished by a single operator without the need for hoisting equipment.
In order for the fixture to be rotated and translated from one position to another, the table is provided with one or more retractable pivot pins which project upwardly from the table surface. The pins may either be received in a socket in the bottom of the fixture, in which case the fixture is constrained to move circularly on the table, or, alternatively, they may be received in one or more slots so that the fixture is not only rotatable but also translatable. Cooperating pairs of retractable pin and socket locating devices on the fixture and table provide for accurate location of the fixture in a variety of predetermined positions. Clamps may also be provided for clamping the fixture in the desired positions during machining. The clamps may either be of the conventional T-slot type, or, alternatively, of the bayonet type as described in U.S. Pat. No. 4,174,828 in the name of Raymond A. Bergman.
The table may be provided with a dual centering pin feature if additional flexibility is desired in positioning and locating the fixture. The dual centering pin comprises an outer pin which is engageable with the fixture slots, and an inner pin received within the outer pin with means for causing the inner pin to extend upwardly out of the outer pin so as to engage holes in the slots. When the inner pin is retracted, the workpiece fixture is translatable on the table in engagement with the outer pin, and when the inner pin is extended and received within one of the fixture holes within the slots, the fixture may be rotated about it to the desired position.
Although the prior developments of the air-float system as described above have greatly facilitated the positioning and repositioning of workpieces within the tool station, it was still necessary for large traveling hoists to be used for transferring the workpiece to the machine tool station, and then removing it after all of the machining steps have been accomplished. Furthermore, multiple machining steps often necessitates the movement of the workpiece to two or more machining stations, which, although they may be in fairly close proximity, are not contiguous so that a hoist must again be used to lift the workpiece and move it to the next station. This results in substantial machine down time because the operator must wait for the hoist to become available to remove the machine part, and then wait again for the hoist to deliver the next part for machining. Another drawback is the inherent danger in lifting and transporting extremely heavy workpieces above the working area.
Although positioning the work tables for adjacent machining stations in close proximity to one another with their upper surfaces contiguous may solve the problem of moving the workpiece from one station to the next without relying on a traveling hoist, such a solution is limited in its applications. For example, when rotary tables are used, it is not possible for adjacent tables to be in close proximity due to the need for clearance. Furthermore, it is often desirable for machining tables to be spaced from one another by an aisle so that other equipment, such as gantry cranes, or the like, may pass therebetween. The use of a very large air float table is inefficient in the situation where only one of the machining stations which it services is used.